Multi-active bridge based DC-link balancing of three-level NPC inverters

Abstract

A common topic in multilevel converter research is the question of DC-link capacitor voltage balancing. For the three-level neutral point clamped converter the balancing is difficult due to the nonlinear nature of the DC-link capacitor voltages. Previous papers solved this problem using sophisticated controllers or additional circuitry connected in parallel to the load. Those solutions however restrict the performance of the output voltage since the controller has to provide a trade-off between output voltage performance and DC-link balancing while the load-connected balancing circuit puts limitations on the applied modulation scheme. In this paper, we connect a multi-active bridge circuit to the DC-link of the converter to overcome this problem. The proposed method is independent of the modulation scheme of the neutral point clamped converter and allows to use of all possible switching states to control the output voltage without taking the effect on the DC-link balance into account. The efficiency of the proposed method is verified using experimental results.